In conventional alpine ski binding designs, release of a ski boot is by means of a toe unit that senses twist and a heel unit that senses upward forces applied by the heel of the boot. No other information is involved in the release/retain logic of the heel unit. The heel unit is intended to help protect a skier from injury caused by an excessive forward bending moment on the skier's lower leg. However, the force applied by the boot heel to the heel unit is not always an indication of the true bending moment on the leg. Inadvertent release of the heel unit among elite skiers and competitors occurs sporadically despite the wide-spread use of release settings well above the recommended safe settings. There are also conditions that can lead to bending-related injury to the lower leg in forward falls even when relatively low release settings are used.
Inadvertent release in the former scenario just mentioned is referred to as the “Bow Effect,” based on the cause of the inadvertent release. This Bow Effect has characteristics similar to the situation in which an archer allows a bow to slip from their grasp while flexing the bow to install a bow string. In this case, energy stored in the flexed bow releases, thereby causing the bow to tend to move in the direction of the end not in contact with the ground. In skiing, the release generally follows the storing of flexural energy in the front portion of the ski in reaction to a bump or rut. When this stored flexural energy is released, it tends to propel the ski rearward relative to the boot. At any time the distributed load applied by the snow to the ski can be represented by a single vector. This vector is generally perpendicular to the bottom surface of the ski in the vicinity of the ball of the skier's foot. However, when the ski encounters a bump or rut, this vector moves forward and away from the ski boot. In the situation described as the Bow Effect, the vector has a large component parallel to the long axis of the sole of the ski boot, and if the skier does not have most of their bodyweight on that ski, the vector has a small component perpendicular to that axis.
In skiing, the moment experienced by the binding is calculated by multiplying the magnitude of the force vector on the ski by the perpendicular distance to the pivot point (fulcrum) between the boot and binding in a forward lean, whereas the bending moment experienced by the skier's leg at the same moment in time is calculated by multiplying the magnitude of the force vector on the ski by the perpendicular distance to the skier's boot top. Therefore, during a Bow Effect event, the moment experienced by the binding is much greater than the moment experienced by the skier's lower leg. When the binding releases, the lack of pressure between the upper cuff of the boot and the skier's lower leg causes the skier to classify the release as inadvertent and unnecessary. However, the trajectory of the ski in the direction opposite to the skier's direction of travel indicates the cause to be the Bow Effect.
The heel release itself is brought about by the skier driving their lower leg forward at the same time as the flexural energy in the front portion of the ski releases. The inadvertent simultaneity of these phenomena can put the skier's lower leg into tension, thereby pulling the heel unit open with little apparent effort. Increasing the release threshold of the heel unit does not necessarily eliminate the bow effect and, in fact, can cause injury to the skier during situations in which the heel unit should have released but did not because the released threshold was increased in attempt to counter the bow effect.
In 1985, one of the present inventors coauthored a paper titled “A Method For Improvement of Retention Characteristics in Alpine Ski Bindings,” which had been presented at the fifth symposium of the International Society for Skiing Safety in Keystone, Colo., in 1983 and later published by the American Society for Testing and Materials (ASTM) as a special technical paper (STP) in ASTM STP 860. The paper is based on a study that included field observations and laboratory re-creations of the Bow Effect. Using a test method based on ASTM F504, a 50% drop in the measured bending moment on a simulated leg can be shown for the most extreme condition. Comparable increases in the release moment in a forward lean can also be measured in simulated “hard landing” situations following a jump, and in situations in which the skier is falling forward and, at the same time, the ski is going uphill (the tip of the ski is higher than the tail). These observations and re-creations demonstrate the generally unreliability of the traditional heel binding unit under extreme, but foreseeable, conditions. Among racers of all levels, as well as among some experienced recreational skiers, the problem has lead to a general loss of confidence in the sanctioned method for release threshold selection. What is needed, therefore, is an alpine ski binding that improves the release/retention performance of the heel unit of the binding by increasing or decreasing the force required to release the heel of a ski boot as required during skiing so that, at the release threshold, the bending moment on the leg is approximately the same.
ASTM F504-05 test 2.3 simulates a slow weighted forward fall and uses a load point on the ski defined as the “near point.” On level ground it is the approximate balance point when a typical skier leans forward to the average limit of dorsiflexion. ASTM F504 does not define a test with a load point any closer to the boot. However, Sub-Near-Point loads are possible in alpine skiing when a skier falls forward as a ski encounters a rut or bump with a sharp uphill transition. As the ski encounters the steep transition, it is at first decelerated,which can throw an unprepared skier forward. Then, as the ski rides up the slope of the obstruction and the portion of the ski under the boot enters the transition, the skier's boot and lower leg experience a rapid angular acceleration as the boot toe rotates upward. This motion can snap the knee joint of the unprepared skier (who is already falling forward) into full extension. The ski and boot then accelerate upward relative to the skier's center of gravity, creating a more than one-g loading environment for the lower leg. At injury, the resultant force vector on the ski is closer to the boot than the ASTM-defined near-point and has a small component in the direction of the long axis of the boot pushing the ski forward away from the boot and a large component perpendicular to the long axis of the boot. In this situation, the perpendicular distance from the resultant force vector on the ski to the pivot point of the binding is much shorter than the distance to the boot top. Therefore, the leg experiences a much greater moment than the binding.
In summary, the resultant force vector on the ski during inadvertent release by the Bow Effect is located near the tip of the ski and has a large component parallel to the long axis of the sole of the boot and a small component perpendicular to this long axis. In contrast, the resultant force vector on the ski during injury due to Sub-Near-Point loading is located closer to the boot than the near-point and has a small negative component parallel to the long axis of the sole of the boot and a large component perpendicular to this long axis. The near-point is located approximately at a distance of 25% of the skier's height forward of the skier's lower leg.